yes, if you measure from hole to hole like you show, that will determine the fan size you need. In the case of the pic, an 80mm (8cm) fan. But keep in mind, the smaller the fan the noisier it will be to move the same amount of air as a larger fan. Case fans aren't expensive, so don't go all frugal when selecting one.
Your graphics card will be a real heat producer when using it at levels of power... like gaming. But it should ventilate itself by pulling in case air and exhausting it out the back of the case. Be sure the card is clean, so it can move air. Blow it out with some canned air. It sounds like you have a small case. I would add a front fan as well. It will push cool air into the case. Use the largest fans you case will accept.
My graphics card doesn't have a proper fan, I went for the PNY model over some other make and the PNY only has some stupid heatsink. And the graphics card is directly above the graphics card, with the heatsink facing the wireless card. I might also remove the PCI wireless though and replace it with a gigabit ethernet card .
I had one of those a while back, it ran at like 90C, I know where you're coming from!
Looks like you can install 80mm or 92mm fans on the back and 80mm on the front.
I would suggest a thinner fan for the front, there won't be too much space.
Be sure to check the airflow direction an the fans when installing. You want the front fan to be pulling air in and the back fan to be pushing it out. Indications on air flow direction are on the one side of the fan. You could also find a splitter cable to connect both to the chassis fan 3pin connector on your mobo.
The two fans will do the job, but it would be better (considering the job they are doing) to find decent quality fans that will be more efficient.